Radio systems are used to transmit and receive signals in wireless communications systems. For example, base stations and other wireless nodes in a cellular communications network are equipped with radio systems that transmit and receive signals. Further, in many implementations (e.g., Frequency Division Duplexing (FDD)), a radio system simultaneously transmits and receives. A radio system that simultaneously transmits and receives must adequately isolate the high power transmitter from the sensitive receiver. This isolation can be obtained via separate antennas for the transmitter and the receiver or via some isolating device (e.g., a circulator or duplex filter) that duplexes the transmitter and the receiver onto the same antenna. Inadequate isolation will result in significant leakage of the transmit signal into the receiver, which can potentially desensitize the receiver.
There is a need for techniques for improving the isolation between the transmitter and the receiver in such a radio system that do not rely on incremental advancements in passive isolation (e.g., better circulators or better duplex filters). In this regard, some active cancellation techniques have been previously disclosed to cancel the portion of the transmit signal that leaks into the receiver. In particular, W. Schacherbauer et al., “An Interference Cancellation Technique for the Use in Multiband Software Radio Frontend Design,” 30th European Microwave Conference, October 2000, pages 1-4 (hereinafter “Schacherbauer”) teaches an active cancellation technique in which an auxiliary transmit chain generates a Radio Frequency (RF) cancellation signal that is used to cancel the transmitter leakage in the receiver. This technique improves isolation only in the transmit frequency band. More specifically, FIG. 1 illustrates a radio system 10 that includes an active cancellation system 12 as described in Schacherbauer. As illustrated, in addition to the active cancellation system 12, the radio system 10 includes a transmitter 14 including a Power Amplifier (PA) 16 and a receiver 18 including a Low Noise Amplifier (LNA) 20. The output of the transmitter 14 and the input of the receiver 18 are coupled to an antenna 22 via a duplexer 24 (e.g., a circulator or duplex filter).
During operation, a portion of the transmit signal output by the transmitter 14 leaks through the duplexer 24 into the receiver 18. The portion of the transmit signal that leaks into the receiver 18 is referred to as a transmit leakage signal. The active cancellation system 12 includes a digital Finite Impulse Response (FIR) filter 26 and an auxiliary transmitter 28 that operate to generate an RF cancellation signal. The RF cancelation signal is combined with the RF receive signal prior to entry to the receiver 18 via a coupler 30. The digital FIR filter 26 is designed to have a transfer function that models the leakage path from the output of the transmitter 14 through the duplexer 24 to the input of the receiver 18, including antenna reflections.
During training, a wideband training sequence is sent through the transmitter 14. This training sequence is used to calculate the coefficients of the digital FIR filter 26. Using the calculated coefficients, the digital FIR filter 26 operates such that the RF cancellation signal cancels (i.e., minimizes or otherwise mitigates) the transmit leakage signal. In doing so, isolation between the transmitter 14 and the receiver 18 is improved. In Schacherbauer, the test setup included a fixed wideband duplexer that provided 25 decibels (dB) of isolation between the transmitter 14 and the receiver 18. For a transmit signal with a 5 Megahertz (MHz) bandwidth centered at 1.96 Gigahertz (GHz), the additional isolation provided by the active cancellation system 12 was 35 dB.
S. Kannangara et al., “Adaptive Duplexer for Multiband Transreceiver,” Proceedings of the 2003 Radio and Wireless Conference, Aug. 10-13, 2003, pages 381-384 (hereinafter “Kannangara”) teaches another active cancellation technique. In particular, Kannangara teaches an active cancellation system that utilizes a feed-forward technique to enhance a fixed duplexer by improving isolation in both the transmit frequency band and the receive frequency band. FIG. 2 illustrates a radio system 32 that includes an active cancellation system 34 that implements the feed-forward technique of Kannangara. As illustrated, in addition to the active cancellation system 34, the radio system 32 includes a transmitter 36 including a PA 38 and a receiver 40 including an LNA 42. The output of the transmitter 36 and the input of the receiver 40 are coupled to an antenna 44 via a duplexer 46 (e.g., a circulator or duplex filter).
During operation, a portion of the transmit signal output by the transmitter 36 leaks through the duplexer 46 into the receiver 40. The active cancellation system 34 includes a coupler 48, a splitter 50, fixed delays 52 and 54, a pair of vector attenuators 56 and 58, a combiner 60, and another coupler 62. The coupler 48 taps the output of the transmitter 36. The resulting signal is then split by the splitter 50 to provide two feed-forward signals. The feed-forward signals are passed through the corresponding delays 52 and 54 and vector attenuators 56 and 58 to provide delayed and attenuated feed-forward signals. The delayed and attenuated feed-forward signals are combined by the combiner 60 to provide an RF cancellation signal, which is then combined with the RF receive signal via the coupler 62. The vector attenuators 56 and 58 are used to adjust the phase and amplitude of the delayed feed-forward signals to provide the desired cancellation. In Kannangara, measured results were published with a transmit frequency band centered at 1955 MHz and a receive frequency band centered at 2145 MHz. The duplexer 46 used for the measurements in Kannangara provided at least 20 dB of isolation in both bands. The active cancellation system 34 increased the isolation in the transmit frequency band by 47 dB and increased the isolation in the receive frequency band by 38 dB. The attenuation was measured over 5 MHz channel bandwidths.
M. Knox, “Single Antenna Full Duplex Communications Using a Common Carrier,” 2012 IEEE 13th Annual Wireless an Microwave Technology Conference, Apr. 15-17, 2012, pages 1-6 (hereinafter the “Knox Article”) and U.S. Pat. No. 8,111,640 B2 entitled “Antenna Feed Network for Full Duplex Communication” to Knox, which was filed on Jul. 10, 2009 and issued on Feb. 7, 2012 (hereinafter the “Knox Patent”) teach another active cancellation technique. In the Knox Article and the Knox Patent, the transmit signal at the output of the transmitter is split into two paths using a hybrid coupler. The split transmit signals are orthogonal due to the −90° phase shift in one path of the hybrid coupler. The split signals are then fed to an antenna that has two feed-points that are intended for orthogonal signals. An example is an antenna that sends/receives a circularly polarized signal by transmitting two linearly polarized signals that are perpendicular where the signals to each polarization have a phase difference of 90°. Another hybrid coupler is used in the receive path. Due to the phase shifts of the hybrid couplers, the transmit leakage signal is destructively combined before going into the receiver, thereby reducing leakage and improving isolation.
The advanced cancellation technique of the Knox Article and the Knox Patent is implemented in a radio system 64 of FIG. 3. The radio system 64 includes a transmitter 66 including a Digital-to-Analog Converter (DAC) 68 and PA 70, and a receiver 72 including an LNA 74 and an Analog-to-Digital Converter (ADC) 76. The output of the transmitter 66 and the input of the receiver 72 are coupled to an antenna 78 via a duplexer system 80. The duplexer system 80 includes a first hybrid coupler 82, circulators 84 and 86, and a second hybrid coupler 88. In operation, an RF transmit signal is output from the transmitter 66 to a first port 90 of the first hybrid coupler 82. The first hybrid coupler 82 applies an approximately −90° phase shift to the RF transmit signal to provide a first phase-shifted RF transmit signal at a second port 92 of the first hybrid coupler 82 and applies an approximately 0° phase shift to the RF transmit signal to provide a second phase-shifted RF transmit signal at a third port 94 of the first hybrid coupler 82. Ideally, the two phase shifts are exactly −90° and 0°, respectively. However, due to, e.g., manufacturing tolerances, the phase shifts may vary from the ideal case (e.g., −88° and 2°).
The first phase-shifted RF transmit signal is provided to a first port 96 of the circulator 84. Due to the normal operation of the circulator 84, the circulator 84 passes the first phase-shifted RF transmit signal from the first port 96 of the circulator 84 to a second port 98 of the circulator 84. However, a portion of the first phase-shifted RF transmit signal (i.e., a first phase-shifted transmit signal) leaks from the first port 96 of the circulator 84 to a third port 100 of the circulator 84. In a similar manner, the second phase-shifted RF transmit signal is provided to a first port 102 of the circulator 86. Due to the normal operation of the circulator 86, the circulator 86 passes the second phase-shifted RF transmit signal from the first port 102 of the circulator 86 to a second port 104 of the circulator 86. However, a portion of the second phase-shifted RF transmit signal (i.e., a second phase-shifted transmit signal) leaks from the first port 102 of the circulator 86 to a third port 106 of the circulator 86.
In this case, the first and second phase-shifted transmit signals provided from the circulators 84 and 86 are 90° out-of-phase (i.e., are orthogonal). The first and second phase-shifted RF transmit signals are provided to two orthogonal feed-points of the antenna 78. Notably, while FIG. 3 illustrates two antennas, only one antenna is used, but with two orthogonal feed-points. As such, the antenna 78 functions as two separate antennas with different properties (e.g., different polarizations).
During reception of an RF receive signal, the first phase-shifted transmit leakage signal at the third port 100 of the circulator 84 enters a first port 108 of the second hybrid coupler 88. Similarly, the second phase-shifted transmit leakage signal at the third port 106 of the circulator 86 enters a second port 110 of the second hybrid coupler 88. At this point, the first phase-shifted transmit leakage signal is approximately −90° out-of-phase with the second phase-shifted transmit leakage signal. The second hybrid coupler 88 applies an approximately −90° phase shift to the first phase-shifted transmit leakage signal when passing the first phase-shifted transmit leakage signal from the first port 108 of the second hybrid coupler 88 to a third port 112 of the second hybrid coupler 88, and applies an approximately 0° phase shift to the second phase-shifted transmit leakage signal when passing the second phase-shifted transmit leakage signal from the second port 110 of the second hybrid coupler 88 to the third port 112 of the second hybrid coupler 88. Due to the phase shifts applied by the first and second hybrid couplers 82 and 88, the first and second phase-shifted transmit leakage signals are approximately 180° out-of-phase at the third port 112 of the second hybrid coupler 88. Therefore, the first and second phase-shifted transmit leakage signals destructively combine at the third port 112 of the second hybrid coupler 88. In this manner, the transmitter leakage is cancelled, or mitigated, before entering the receiver 72. Further, during transmission, signals that are reflected back from the antenna 78 will also experience some destructive combining prior to going into the receiver 72, thereby reducing leakage due to antenna reflections.
The author of the Knox Article is also an inventor listed on U.S. Pat. No. 8,077,639 B2, entitled “High Isolation Signal Routing Assembly for Full Duplex Communication,” which was filed on Jun. 29, 2009 and issued on Dec. 13, 2011 (hereinafter the “Second Knox Patent”). A representative figure for the teachings of the Second Knox Patent is illustrated in FIG. 4. Notably, FIG. 4 is substantially the same as FIG. 3 but where the dual-input antenna 78 is replaced with a hybrid coupler 114 and an antenna 116 with a single input. Otherwise, the radio system 64 of FIG. 4 operates the same as that described above with respect to FIG. 3, particularly with respect to cancellation of the transmitter leakage.
Each of the active cancellation techniques above has its own associated problems. In particular, the active cancellation technique of Schacherbauer and illustrated in FIG. 1 uses a cancellation signal that is a linear function of the baseband digital signal. The cancellation signal cannot cancel Intermodulation Distortion (IMD) or noise generated by any of the transmitter components. Furthermore, any noise or IMD generated by the auxiliary transmitter 28 in the receive frequency band decreases the sensitivity of the receiver 18. Large attenuation performance is limited to transmitters with low noise and highly linear components.
The active cancellation technique of Kannangara was designed for a mobile terminal, not a base station. The dynamic range of the base station transmit signal between the transmit frequency band and the receive frequency band is much larger than that of mobile terminals. As such, implementing the active cancellation technique of Kannangara in a base station would require vector modulators with the same dynamic range in the feed-forward paths, which is infeasible for typical base station requirements.
The active cancellation technique of the Knox Article, the Knox Patent, and the Second Knox Patent is limited by component tolerance and frequency variation (i.e., gain and phase shift of the hybrid couplers). Furthermore, the Knox Article and the Knox Patent require an antenna with two orthogonal input feed points. The reflections from these feed points are assumed to be equal to realize cancellation of the antenna reflections. So, the antenna design is further constrained to ensure this assumption applies.
In light of the discussion above, there is a need for a duplexer system providing improved transmitter-receiver isolation.